1. Field of the Invention
The present invention concerns motion picture film projection. More particularly, the present invention concerns a film tension sensing device for a motion picture film projector. Even more particularly, the present invention concerns a film tension sensing device that is deployed with a platter feed system operating in conjunction with a motion picture film projector.
2. Description of the Prior Art
The motion picture industry began showing motion picture films to the public initially by use of the carbon arc projector. This machine operated by supplying an electrical current through two carbon rods. Ignition between the carbon rods occurs, supplying a bright light, which can be focused to facilitate the projection of film images.
One of the drawbacks of the carbon arc projector is the mess associated with the burning of the carbon rods. The next advance was the development of the xenon bulb. The bulb facilitated the cleaner operation of cleaner projector equipment. It also lessened the demands placed upon a projector technician during operation.
To this point, the motion picture was displayed by a pair of projectors. While the first projector displays one reel, a second projector is prepared with the next reel to be shown. When the operator sees a cue in the upper right corner of the screen, he starts the second projector and halts the first projector. Thus, a smooth change between machines is effected.
Market demands to improve the product offered to the viewing public and the need to reduce costs prompted the elimination of the dual projector format. This was accomplished by the development of the platter system. The platter system comprises at least two, and commonly three, large rotatable platters. The feature film to be shown, which is contained on several separate reels, is spliced together to form one continuous piece of film. This film is laid on one of the platters, from which it is drawn over pulleys and rollers to the projector. The film is then collected on a second platter, where it is then ready for the next showing, only needing to be re-threaded to the projector. The manifest benefits of the platter system has led to its market dominance in the domestic theaters.
As platter systems gained acceptance, it became known that such systems were prone to a problem called headwrap. This problem occurs by the film becoming tangled around the feed-out rollers on the platter. This would result previously in the breakage of the film, requiring a hasty repair by the technician while patrons waited.
More recently, the film industry has moved to making film from polyester. Such films are much less prone to breakage, as opposed to previous films. Thus, instead of the film receiving the damage in a headwrap situation, the equipment suffers damage. In extreme cases, the damage can render the equipment inoperable and present a premature ending to a film presentation, necessitating refunds and incurring public complaint.
One solution to this problem was to have a device cut the film if a headwrap situation was detected. This is obviously an unworkable situation, which seeks only to lessen the damage to be encountered in a headwrap.
A more reasonable attempt to address this problem of headwrap in a platter feed system is found in U.S. Pat. No. 5,189,451 issued to Bredin. Bredin teaches a tension sensing device in communication with an electrical interrupt circuit. A spring holds a micro-switch in tension. A roller of the device has the film wound there around. If the tension increases, the roller lifts against the spring, activating the micro-switch. The micro-switch, connected by relay to the projector's power supply, interrupts the projector operation.
The Bredin device, however, has several shortcomings. The interruption of power, due to weather conditions, or irregularities in supply from the utility, can cause surges. Since the Bredin device automatically resets itself after power outages, intermittent power interruptions can result in the headwrap and associated damage to the machine. Also, when film tension is removed, the Bredin device resets itself. Thus, safety is sacrificed in such circumstances.
Additionally, the new polyester films often stick together. This may be due to the sticky residues associated with splicing, or to static electricity. Films may also not be dry when first produced, which is called green or wet film. Any of these situations can cause a momentary increase in film tension, which poses no threat to the equipment. Yet the Bredin device will interrupt operation of the projector.
Additionally, the Bredin device itself can be damaged in a headwrap condition. The Bredin device utilizes a spring. This member will fatigue and fail over time. An extreme headwrap can hasten this process. The device is formed of a thin gauge of aluminum, which could be deformed by the tension of a headwrap. Thus, the Bredin device could itself conceivably interrupt a motion picture presentation.
Thus, there is a need in the art for a device to detect excess tension in the film feed of a motion picture projector. Further, this device must be capable of detecting true headwrap conditions from momentary tension increases due to other circumstances. Additionally, a tension detection device is desired that is durable and resistant to damage.
It is to these needs that the present invention is directed.